Date Code 20001006 Trip and Target Logic 5-3
SEL-351 Instruction Manual
Set Trip
Refer to Figure 5.1. All trip conditions:
• Communications-Assisted Trip
• Direct Transfer Trip
• Switch-Onto-Fault Trip
• Other Trips
are combined into OR-1 gate. The output of OR-1 gate asserts Relay Word bit TRIP to logical 1,
regardless of other trip logic conditions. It also is routed into the Minimum Trip Duration Timer
(setting TDURD).
As shown in the time line example in Figure 5.2, the Minimum Trip Duration Timer (with setting
TDURD) outputs a logical 1 for a time duration of “TDURD” cycles any time it sees a rising edge
on its input (logical 0 to logical 1 transition), if it is not already timing (timer is reset). The
TDURD timer assures that the TRIP Relay Word bit remains asserted at logical 1 for a minimum
of “TDURD” cycles. If the output of OR-1 gate is logical 1 beyond the TDURD time, Relay
Word bit TRIP remains asserted at logical 1 for as long as the output of OR-1 gate remains at
logical 1, regardless of other trip logic conditions.
The Minimum Trip Duration Timer can be set no less than 4 cycles.
Figure 5.2: Minimum Trip Duration Timer Operation (See Bottom of Figure 5.1)
Note: OPEN Command no longer embedded in Trip Logic
In previous firmware versions of the SEL-351 Relay, the OPEN command was embedded
in the trip logic in Figure 5.1. The OPEN command was routed directly into the Minimum
Trip Duration Timer, along with the output of logic gate OR-1.
Courtesy of NationalSwitchgear.com
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